The present invention relates generally to ophthalmic lenses, and more particularly to improved ophthalmic no-line progressive addition lenses for spectacles.
It is known that as a person ages their eyes' accommodative ability decreases. This condition is called presbyopia and most people become symptomatic for presbyopia between 40 and 45 years of age. At present, the only way to relieve this condition is to employ the use of lenses with either the entire lens or a segment thereof having a more convex front surface, known in the optical industry as plus addition, near addition or add.
Multifocal lenses such as bifocal and trifocal lenses have been devised to aid the vision of persons who suffer from reduced powers of accommodation. The bifocal lens, for example, is in effect formed from two separate segments of different dioptric powers. The power of one segment is such that vision through it permits focusing on near objects such as reading matter. The other segment corrects the vision for viewing distant objects.
There have been many lined bifocal lens designs from the Ben Franklin bifocal of 1785 to the present day lined bifocals. An abrupt change in curvature on the front surface of these bifocal lenses creates the lines and produces undesirable optical and cosmetic effects.
Attempts have been made in the past to eliminate lines of optical demarcation in bifocal and trifocal lenses. One solution to this problem is to provide an intermediate region between the near and far vision segments having properties that vary gradually with distance over the surface so as to provide a gradual transition between the near and distance portions of the lens. Such lenses are frequently referred to as progressive addition lenses. Furthermore, because of the age related stigma that is associated with the use of lined bifocals, the demand for improved no-line progressive addition lenses has significantly increased in recent years.
Known progressive addition lenses have aspheric front surfaces that are ellipsoidal or resemble ellipsoidals. It has been suggested that these aspheric ellipsoidal surfaces are made by adjoining conic sections, which may more closely resemble parabolas or hyperbolas, of different elliptical curves to create the increasing convexity desired for the near add.
While known progressive addition lenses are advantageous in providing variable focusing powers as one looks down the lens, they are disadvantageous because the intermediate and near addition zones are small in width and are surrounded by areas of moderate to high unwanted cylinder values, which the optical industry refers to as astigmatism. Astigmatism is what creates large areas of blur and distortion seen in many of today's known progressive addition lenses. Although the optics of known progressive addition lenses may be altered to make hard and soft designs, mono and multiple designs, and symmetric or asymmetric designs, each lens typically has the same problem of narrow intermediate corridors and small near addition zones with large areas of blur and distortion.
There have been few major advances in the designs of progressive addition lenses in the last 10 to 15 years, and thus, there are limitations with respect to known progressive addition lens designs and theories. A first limitation is there needs to be an infinite number of dioptric changes in the intermediate zone for the patient to see clearly at all distances. It is well known that a person's pupils get smaller with age, thereby allowing the person to have an increased depth of field and depth of focus. Also, presbyopia does not imply that all accommodative ability is suddenly lost. On the contrary, many people can still accommodate 0.25 to 0.50 diopters in their seventh to eighth decade of life. By making the intermediate zone with a finite number of addition steps the dimensions of the intermediate zone(s) and near zone can be greatly increased without compromising clear vision. The dioptric changes can also be done in small enough steps that it will yield little visual confusion as the patient moves down the lens from one addition zone to the next. Therefore, increasing the usable dimensions of the intermediate zone(s) would be very advantageous because the demand for intermediate reading, i.e., computers, is increasing and patients need the intermediate powers for longer periods of time.
Another limitation is theories currently employed for designing the ellipsoidal surfaces are comparable and the only known way that a no-line progressive addition lens can be made. Therefore, it would be desirable to provide a lens which yields a substantially different aspheric front surface and which has improved qualities from known progressive addition lenses.
A further limitation is there must be no definite areas of power change in the distance-intermediate or intermediate-near junction zones. While an infinite number of dioptric changes does yield a smooth progression it also restricts the dimensions of the intermediate and near zones and creates distortion in the peripheral area of known progressive addition lens designs. Hence, it would be desirable to have a lens with small power changes in these zones without causing significant visual disturbance and such that the patient can easily adapt.
As will become more apparent from the discussion which follows, many disadvantages of known progressive addition lenses have been effectively overcome by the unique lenses of the present invention. Furthermore, in a highly novel manner, embodiments of the present invention provide no-line progressive addition lenses which reduce areas of distortion, increase the usable size of the intermediate addition zone(s), and increase the usable size of the near addition zone.